Low molecular weight, soluble factors as well as extracellular matrix molecules play major roles in the growth and development of all tissues. This includes normal tissue and tumor tissue. Laminin may play such a critical role in retinal development. Insulin and especially IGF-l may act as messengers coding for differentiation in the retina and, by affecting phosphorylation of the G-protein transducin, may be directly or indirectly involved in the visual process. Abnormal protein kinase activity and thus cyclic AMP function may be involved in the rapid, uncontrolled growth of retinoblastoma tumor cells.